1. Field of the Invention
This invention relates to earth boring tools and in particular to earth boring tools providing steering of a drill head while the head is rotating.
2. Related Art
The use of horizontal drilling rigs for drilling holes generally following the surface of the earth for installation of underground utilities has become well established in the last ten years. Originally, underground utilities were installed by placing a utility string of pipes, conduits, communication lines, or power cables in an open trench. Devices have also been developed that drill or push a drill string horizontally. The drill string is then used to guide boring equipment which pulls the desired utility string into the resultant borehole. This allows installation of the underground string without disturbing the earth surface between the ends of the string. These early devices are unguided.
The need for curving the underground bore in order to avoid underground and surface structures and generally control the path of the borehole soon became clear. This was provided by the use of special drill heads that could be externally controlled to bend, and thereby change the direction of the drill head.
Another scheme developed involves the use of a drill bit that is longitudinally slanted or wedge-shaped. When it is desired to change direction, the rotation of the drill head is stopped, the head is oriented to produce movement in a desired diverging direction, and the drill head is pushed into the earth. This changes the direction of the drill head. When rotation of the drill head resumes, the drill head is travelling in an altered direction. When performed repeatedly, the drill string can be directed to follow a curved path.
Such a system is effective for drilling through soil, but is not effective if the soil is quite soft or if rock or other hard material is encountered. Also, the system is relatively slow in that rotation of the drill string, which produces the advance in the borehole, must be stopped while the steering maneuver is performed. The greater the changes in borehole direction that are required, the slower the progress. Additionally, it takes more energy to stop and start rotation since the static friction between the drill string and borehole is greater than the dynamic friction. Further, when the drill string stops, drill shavings tend to impact between the drill string and borehole surface, a phenomena referred to as collaring. This further resists movement of the drill string in the borehole.
Such systems are also clearly not suitable for any but the most basic soil conditions. If a hard material such as rock is encountered or if particularly soft material such as sand is encountered, then this technique cannot be used. There thus is a need for a simple horizontal drilling system that overcomes these disadvantages of conventional drilling techniques.